1. Field of the Invention
The present invention relates to an image forming apparatus. More particularly, the present invention relates to a fixing device for an image forming apparatus, for fusing a predetermined toner image transferred onto a printing medium by heat and pressure.
2. Description of the Related Art
General image forming apparatuses such as a copy machine, a printer, a facsimile and a multi-function machine which compositely comprises functions of the preceding apparatuses, print out an input image signal onto a printing medium.
Such an image forming apparatus comprises a developing unit, a transfer unit and a fixing unit. FIG. 1 shows an example of the fixing unit which fuses a transferred toner image onto the printing medium by the transfer unit using heat and pressure.
Referring to FIG. 1, the fixing unit includes first and second fixing rollers 10 and 20 operating in rolling contact with each other by a predetermined fixing nip.
The first fixing roller 10 includes therein a halogen lamp 11. The halogen lamp 11 heats the first fixing roller 10 up to a predetermined temperature (hereinafter, referred to as a “fixing temperature”) for an image fixing operation and maintains the fixing temperature. As a result, the toner image transferred to the printing medium P is properly heated to be fused as passing through a space between the first and the second fixing rollers 10 and 20.
The second fixing roller 20 is biased toward the first fixing roller 10 by an elastic member 21 to fuse the toner image on the printing medium P which is heated by the first fixing roller 10.
However, the general fixing unit of an image forming apparatus, as described above, is structured in a manner that the heat generated by the halogen lamp 11 disposed at the center portion inside the first fixing roller 10 is radiantly transmitted to the first fixing roller 10, thus heating the first fixing roller 10 throughout. Therefore, the first fixing roller 10 may be unnecessarily heated even at a portion thereof which would not contact the printing medium P, accordingly causing loss of heat. Furthermore, since the first fixing roller 10 is preheated without contacting the halogen lamp 11, an extended time is taken for the first fixing roller 10 to reach the fixing temperature. In other words, a warming-up time of the first fixing roller 10 is elongated.
The warming-up time may be reduced by heating the first fixing roller 10 by an induction heat from a magnetic field generated at an induction coil upon application of voltage and a resistance heat from a resistant load of the induction coil itself. To overcome the problem, research for reducing the warming-up time noted above is in active progress.
However, when heating the first fixing roller 10 using the induction coil, the magnetic field generated at the induction coil is concentrated at the opposite ends of the first fixing roller 10. Such a density difference between a middle portion and the end portions of the first fixing roller 10 results in unevenness of the fixing temperature.